The present invention relates to an electroluminescent device made of an organic light emitting material, which is used in a light emitting display, a light emitting diode and a surface light emitting source.
An electroluminescent device (EL) is a device utilizing a phenomenon of light emission when electric field is applied to a certain light emitting material. A display made of the electroluminescent device can realize a thin type display with wide angle of visual field and low dissipation power because it is capable of spontaneously emitting light and no back light is required. The display also has a feature that it can perform high-speed response. However, since the light emitting efficiency of a conventional light emitting material is low, luminance of the display was insufficient. There was also a problem that it was difficult to realize a color display because light emitting color of the light emitting material was limited.
Recently, an excellent electroluminescent device using an organic compounds as a constituent material has been reported (e.g. related paper, Applied Physics Letters, vol.51, 1987, p.913). In this report, C. W. Tang et al. disclose an injection type electroluminescent device having a structure that an organic light emitting layer and an electric charge transporting layer are laminated. In case of this electroluminescent device, an excellent electroluminescent device having high luminance is obtained by using a tris(8-quinolinol)aluminum complex (hereinafter abbreviated to Alq) having both high light emitting efficiency and high electron transporting ability as the light emitting material.
In Journal of Applied Physics, vol.65, 1989, p. 3610, it is reported that an organic light emitting layer is formed with Alq which doped with a fluorescent dye such as coumarin derivative, DCM1 (Eastman Chemicals) or the like, and a light emitting device using that organic layer is made and a luminescent color of the device changes by proper selection of the dye. It is also disclosed that the light emitting efficiency is enhanced compared with a non-doped device.
Various researches and developments have been made subsequent to this study. As a result, a fluorescent metal chelete complex, an electron transporting organic molecule and a hole transporting organic molecule have been developed as a new light emitting material and a development of a light emitting material for color display having high luminance has rapidly been advanced.
Various electrode materials suited for these light emitting materials have also been studied. For example, Japanese Patent laid-Open Publications Nos. 60-165771 and 5-121172 disclose a metal thin film electrode made of metals having a small work function such as Mgxe2x80x94Ag, Ca, Ag, Lixe2x80x94Al, Lixe2x80x94Ag, Al and the like as an electron injection electrode, which is formed by deposition.
However, the above electroluminescent device has a problem that life is short. Since the light emitting material and electrode material are easily affected by water, these materials are considerably deteriorated by moisture absorption of the device. Electroluminescence essentially occurs by electronic conduction in the light emitting material, but in the presence of water, conduction of ions simultaneously occurs. When the light emitting efficiency is lowered by such phenomena, a driving voltage of the electroluminescent device is increased and luminance is lowered. As a result, a dark spot is formed at the portion where luminance is considerably lowered.
Various display panel structures for color display have also been studied, together with the development of the material As a structure of a color panel, a structure wherein light emitting layer patterns of three colors are arranged on one substrate is most popular. However, since this structure requires three patterning processes for forming light emitting layer patterns on one substrate, there can be caused problems such as deterioration of each light emitting material due to chemicals and water during the patterning processes, and low yield of the process. To avoid these problems, Japanese Patent Laid-Open Publication No. 7-114350 discloses a color panel structure that three monochromatic panels are superposed. In this publication, each monochromatic panel comprises a transparent made of an acrylic resin or glass substrate. However, a color display panel having this structure is very thick and has a problem that, since adjacent picture element electrodes are separated each other in back and forth, light shadow is formed by each of picture element electrodes and, therefore, resolution and color separation at the time of diagonally observing the picture plane are insufficient and thus high picture quality is not obtained. There is also a problem that the substrate is liable to be broken by distortion on lamination when the substrate is made of glass.
The present invention has been developed to overcome the above described disadvantages.
A first object of the present invention is to solve the above conventional problems by improving a transparent substrate and a transparent electrode, and to provide an injection type excellent electroluminescent device which hardly causes change in luminance with a lapse of time and has few dark spots.
A second object of the present invention is to provide an injection type electroluminescent device which hardly causes change in luminance with a lapse of time and has few dark spots, and which is thin and flexible, by using a new sheet material as a transparent substrate.
A third object of the present invention is to provide an excellent color electroluminescent display panel which hardly causes change in luminance with a lapse of time and has few dark spots, and is thin in thickness, superior in resolution and color separation and easily produced.
In accomplishing the first object, a first aspect of the present invention is an electroluminescent device comprising a transparent anode formed on a transparent substrate; a thin film cathode containing an alkali metal or an alkali earth metal; an electron transporting organic molecule and a hole transporting organic molecule between a pair of electrodes consisting of said transparent and said thin film cathode; and a barrier sheet for covering the outer surface of said thin film cathode; wherein both of the surface of said transparent substrate and the surface of said transparent anode have a hydrophobic surface with a molecular structure without having active hydrogen. With this construction, the water content of the device is reduced from the beginning and, at the same time, penetration of water is inhibited, thereby making it possible to obtain an excellent electroluminescent device which hardly causes change in luminance with a lapse of time and has few dark spots.
In accomplishing the second object, a second aspect of the present invention is an electroluminescent device comprising a transparent anode formed on a transparent substrate; a thin film cathode containing an alkali metal or an alkali earth metal; an electron transporting organic molecule and a hole transporting organic molecule between a pair of electrodes consisting of said transparent and said thin film cathode; and a barrier sheet for covering the outer surface of said thin film cathode; wherein said transparent substrate comprises:
(a) a two-layer polymer film comprising an adhesive hydrophobic polymer layer containing at least one functional group selected from the group consisting of amino group, amide group, hydroxyl group and carboxyl group in the concentration of not more than 1% by weight and a hydrogen-bonding polymer layer containing at least one functional group selected from the group consisting of amino group, amide group, hydroxyl group and carboxyl group, or
(b) a three-layer polymer film comprising said hydrogen-bonding polymer layer, a hydrophobic polymer layer made of at least one kind selected from the group consisting of polyolefin, polyvinylidene chloride, polyvinyl chloride, polyacrylonitrile, fluorine-containing polymer, liquid crystalline polyester and silicone resin and an adhesive polymer layer inserted therebetween containing at least one functional group selected from the group consisting of amino group, amide group, hydroxyl group and carboxyl group.
Regarding the second aspect of the present invention, the transparent substrate is made of an excellent polymer sheet having both barrier property to water and interlaminar bonding property, and the following advantageous effect can be obtained.
(1) Permeation of a gas, especially moisture, from the outside can be remarkably inhibited.
(2) The humidity in the device can be considerably reduced because of no adsorbed moisture on the substrate and no source for evolution of moisture.
(3) The sufficiently dried hydrogen-bonding polymer layer also has an function as a getter of moisture.
(4) The polymer film has a multi-layer construction, in which peeling does not arise because of good interlaminar adhesive property.
(5) Since the transparent substrate is made of a flexible polymer sheet having high transparency, a flexible device having high performance and utility can also be produced.
(6) Although the film has a multi-layer construction, continuous production of the film can be easily performed.
With such a lot of effects, there can be constructed an excellent electroluminescent device which hardly causes change in luminance with a lapse of time and has few dark spots, and which is thin in thickness and flexible.
In accomplishing the third object, a third aspect of the present invention is a multi-color electroluminescent display panel comprising at least two monochromatic panel units,
wherein each monochromatic panel unit comprises (A) a transparent substrate comprising:
(a) a two-layer polymer film comprising an adhesive hydrophobic polymer layer containing at least one functional group selected from the group consisting of amino group, amide group, hydroxyl group and carboxyl group in the concentration of not more than 1% by weight and a hydrogen-bonding polymer layer containing at least one functional group selected from the group consisting of amino group, amide group, hydroxyl group and carboxyl group, or
(b) a three-layer polymer film comprising said hydrogen-bonding polymer layer, a hydrophobic polymer layer made of at least one kind selected from the group consisting of polyolefin, polyvinylidene chloride, polyvinyl chloride, polyacrylonitrile, fluorine-containing polymer, liquid crystalline polyester and silicone resin and an adhesive polymer layer inserted therebetween containing at least one functional group selected from the group consisting of amino group, amide group, hydroxyl group and carboxyl group;
(B) a striped transparent anode on the transparent substrate;
(C) a fluorescent organic layer having an electron transporting organic molecule and a hole transporting organic molecule; and
(D) striped thin film cathodes formed in the direction perpendicular to the striped transparent anode with half or less width of a full width of a picture element of the multi-color electroluminescent device,
and wherein the monochromatic panel units are laminated each other so that said striped thin film cathodes are not overlapped each other above and below.
Since the transparent substrate is made of a polymer film having high barrier property to water, it is possible to construct a long life monochromatic panel unit which hardly causes change in luminance with a lapse of time and has few dark spots. Since the monochromatic panel unit is very thin in thickness, the thickness of the whole panel does not become too large and, furthermore, flatness is sufficient. Since the distance in back and forth between adjacent picture elements is comparatively small, light shadow due to adjacent picture elements is hardly formed and, therefore, resolution and color separation of the display panel are superior. The present invention also has such a feature that, since there is no need to repeat the patterning processes to form light emitting layers of multi colors on one substrate, the panel production steps do not require high precision and are not complicated. Accordingly, the yield of the panel production is high.
As described above, the present invention provides an excellent electroluminescent device which has long life and high picture quality and is easily produced, and its industrial value is great.